Sailboats rely on harnessing the power of the wind to create propulsion. In its most basic form, this is accomplished by affixing a vertical mast to the boat with a sail attached to the length of the mast on one side, and to an adjustable rope on the opposite corner. To support the load on the mast, the mast is connected in a cantilever fashion to the hull. Most sail rigs include a compressive member attached to the lower end of the mast below the sail, extending horizontally to the opposite tip of the sail. This is referred to as a boom. To improve the aerodynamics of the sail, battens are often integrated into the sail cloth. Battens are most commonly constructed of fiberglass rods, and are oriented in a generally horizontal position, spaced out along the vertical length of the sail.
A variation on this conventional rig allows the mast to spin around its vertical axis. This is achieved by supporting a short length of the base of the mast with bearings. Spinning the mast consequently causes the sail to wrap, or furl around the diameter of the mast. This is a benefit to the functionality of the sailboat, allowing the user to furl or unfurl the sail to attain the appropriate amount of sail area for the conditions, or completely furl the sail. Furling the sail around the mast requires the sail to not have horizontal battens, as they are not flexible enough to wrap around the small diameter of the mast. Instead, diagonal battens are used. The compromise to this style of rig is that supporting the mast only at the base compromises the efficiency of the rig because the tip of the mast bends away under the load of the wind, decreasing the amount of power that can be harnessed. It also requires a very strong connection between the hull and the mast, and a stiff mast.
In an effort to increase strength and decrease weight, many sailboats use stays to transmit the load of the mast to the hull of the boat. Stays are the tension members. They are wires or ropes affixed to the mast approximately two-third to three-quarters up the length of the mast. Supporting the mast at this location allows for a balanced wind loading of the mast, minimizing bending and therefore keeping an optimized sail shape. These stays extend down to the left and right sides of the hull and the front of the hull. Using stays to support the mast allows the base of the mast to be simply supported on the hull. It also decreases the strength requirements of the supporting hardware due to the larger moment arm, decreasing the weight of the rig.
In high wind conditions, it can be necessary to decrease the amount of sail area to avoid overpowering or capsizing a sailboat. With a traditional stayed sail rig, the stays are affixed to the mast, precluding the sail from being furled around the mast because it cannot spin. The simplest solution to decreasing sail area with a stayed sail rig is called reefing. In this method, the sail is lowered part way down the length of the mast, and the lower section of the sail is gathered at the bottom of the sail. While this works, it is not very quick or tidy. Another design allows for a boom to spin as the sail is lowered down the length of the mast, furling the sail around the boom. A variation on this design works very similarly, but employs a rotating mandrel inside a hollow boom around which the sail wraps when furled. These two designs require a boom. Another design works in a similar fashion, but furls the sail around a mandrel inside the mast. This design precludes the use of battens and increases the weight of the mast. One more less common design allows for the sail to furl around the mast as it spins, with stays running to the tip of the mast. This is not a common design because only supporting the mast at the top and bottom creates a less optimal sail shape, as the mast bends in the middle. It also makes it difficult to spin the mast under sail load.